should be a 3/8th allen if nobodys messed with it
Try taking a long, very thin screwdriver and/or an awl to clean out the receiving end of the bolt. Blow it out and tap, with a hammer, a long allen wrench into the bolt. A little rust buster also helps. If all that fails, you can cut off the zinc (it's relatively soft) and just push the bolt out. Good luck. Hy